Method for separating hydrogen gas

ABSTRACT

Product gas (Gpro) is separated from material gas (Gmat) by a PSA process utilizing a plurality of adsorption towers (A-C) each loaded with an adsorbent. The separation of the product gas (Gpro) is performed by repeating a cycle comprising an adsorption step, a decompression step, a desorption step, a cleaning step and a pressurization step. In the decompression step, remaining gas (Grem) as cleaning gas is introduced from one adsorption tower (C) to another adsorption tower (B). The amount of the remaining gas (Grem) introduced is 2 to 7 times the volume of the adsorbent loaded in the adsorption tower (B) as converted into volume at common temperature and under atmospheric pressure. To remove both of carbon monoxide and carbon dioxide from the material gas (Gmat) by a single kind of adsorbent, use is made of zeolite having a faujasite structure with a Si/Al ratio lying in a range of 1 to 1.5 and a lithium-exchange ratio of no less than 95%.

TECHNICAL FIELD

[0001] The present invention relates to a gas separation methodutilizing pressure swing adsorption.

BACKGROUND ART

[0002] Various methods for separating target gas (product gas) frommaterial gas are known, and pressure swing adsorption (PSA) is one ofsuch methods. Since a PSA process can be performed easily at arelatively low cost, it is widely utilized in the related field. The PSAprocess typically utilizes a plurality of adsorption towers loaded withan adsorbent. After material gas is introduced in each of the adsorptiontowers, the process steps of adsorption, decompression, desorption andpressurization are repeated to obtain a targeted product gas.

[0003] Specifically, the target gas is obtained on a principle describedbelow. When the pressure of material gas introduced in the adsorptiontower is increased, the partial pressure of an unnecessary gas componentcontained in the material gas also increases. As a result, theunnecessary gas component is adsorbed by the adsorbent loaded in theadsorption tower. (That is, the unnecessary gas component is removedfrom the material gas.) In this state, the gas in the adsorption toweris discharged as target gas (product gas) containing little amount ofunnecessary gas. Thereafter, as the pressure in the adsorption towerdrops, the unnecessary gas component is desorbed from the adsorbent(regeneration of the adsorbent) The desorbed component together withother components remaining in the tower are then discharged from thetower. The regenerated adsorbent can be utilized for removing anunnecessary gas component from newly introduced material gas forobtaining an additional amount of target gas. Examples of target gasesinclude hydrogen gas, oxygen gas and nitrogen gas and the like.

[0004] The kind of adsorbent to be used in each of the adsorbent towersis selected based on the kind of a target gas and the kind of anunnecessary gas component to be removed. For example, zeolite isconventionally used as the adsorbent for removing nitrogen component andcarbon monoxide component from material gas for obtaining hydrogen gasas a product gas. On the other hand, an activated carbon-based adsorbentis used for removing carbon dioxide component from material gas.

[0005] For the PSA process, various improvements have been made forenhancing the purity of the obtained target gas and the yield. Theseimprovements are disclosed in the gazettes of JP-B2-62(1987)-38014,JP-B2-7(1995)-4498 and JP-A-8(1996)-10551.

[0006] As one of the improvements for the PSA process, a technique isdeveloped for increasing the regeneration efficiency of an adsorbent.Specifically, it is now assumed that a desorption step is finished inone adsorption tower (first adsorption tower) while an adsorption stepis being performed in another adsorption tower (second adsorptiontower). At that time, product gas is introduced from the secondadsorption tower to the first adsorption tower. As a result, the gasremaining in the first adsorption tower is discharged, i.e. the firstadsorption tower can be cleaned (cleaning step). Such cleaning canincrease the regeneration effciency of the adsorbent loaded in theadsorption tower, which may result in an increase in the yield of thehydrogen gas.

[0007] Another improvement for the PSA process is as follows. It is nowassumed that an adsorption step is finished in a first adsorption towerand the internal pressure of the adsorption tower is high, while adesorption step (or cleaning step) is finished in a second adsorptiontower and the internal pressure of the adsorption tower is low. In thisstate, the remaining gas is introduced from the first adsorption tower(at high pressure) to the second adsorption tower (at low pressure) forequalizing the internal pressures of the two adsorption towers. Thistechnique is advantageous in that decompression of the first adsorptiontower and pressurization of the second adsorption tower can be performedeasily at the same time.

[0008] The PSA process improved as described above can be performedusing a separation apparatus X as shown in FIG. 1. The separationapparatus X includes three adsorption towers A-C, a material gas pipe 1,a product gas pipe 2, a remaining gas outlet pipe 3, a remaining gasinlet pipe 4, a product purge pipe 5 and a discharge pipe 6. The pipes1-6 are provided with automatic valves a-p. The remaining gas outletpipe 3 and the product purge pipe 5 are provided with flow ratecontrolling valves 7 and 8, respectively. The above-described fiveprocess steps (adsorption, decompression, desorption, pressurization andcleaning) are performed in each of the adsorption towers A-C byselectively opening or closing the automatic valves a-p.

[0009] As shown in FIG. 9, the five process steps are performed in therespective adsorption towers A-C at different timings. In the exampleshown in FIG. 9, nine process steps are defined. For example, in a firststep (S1), an adsorption step (second adsorption step) is performed inthe adsorption tower A, a pressurization step (first pressurizationstep) is performed in the adsorption tower B, and a desorption step isperformed in the adsorption tower C. At that time, each of the automaticvalves (Va-Vp) is open (o) or closed (x).

[0010] The gas flow in the separation apparatus X varies in each processstep. FIGS. 10A-10I illustrate variations of the gas flow. Specifically,as shown in FIG. 10A, in the first step (S1), material gas is introducedinto the adsorption tower A through the material gas pipe 1 and theautomatic valve a. In the adsorption tower A, unnecessary gas componentsare removed by the adsorbent and product gas is discharged from thetower. The product gas is partially collected through the automaticvalve i and the product gas pipe 2 while partially introduced into theadsorption tower B through the product purge pipe 5, the automatic valvep, the flow rate controlling valve 8, the remaining gas inlet pipe 4 andthe automatic valve j. As a result, pressure in the adsorption tower Bis raised. The amount of product gas introduced in the adsorption towerB is controlled by the flow rate controlling valve 8. From theadsorption tower C, the gas remaining in the tower is discharged throughthe automatic valve f and the discharge pipe 6.

[0011] In the second step (S2), an adsorption step (third adsorptionstep), a pressurization step (second pressurization step) and a cleaningstep are performed in the adsorption towers A, B and C, respectively.Specifically, as shown in FIG. 10B, the adsorption step is performed inthe adsorption tower A subsequent to the introduction of material gas.The product gas thus obtained is discharged from the adsorption tower A.The discharged product gas is partially collected while partiallyintroduced into the adsorption towers B and C. The pressure in theadsorption tower B is raised by the introduction of the product gas. Theproduct gas is introduced into the adsorption tower C through theproduct purge pipe 5, the automatic valve p, the flow rate controllingvalve 8, the remaining gas inlet pipe 4 and the automatic valve m. As aresult, remaining gas is discharged from the adsorption tower C. At thattime, it is preferable that the product gas introduced into theadsorption tower C is not discharged and only the remaining gas isdischarged. This is based on the recognition that the collection ofproduct gas is difficult once the product gas is discharged from thetower. In a prior art method, therefore, the amount of product gasintroduced into the adsorption tower C is set to be smaller than thevolume of the adsorbent loaded in the adsorption tower C (as convertedinto volume at common temperature and under atmospheric pressure).

[0012] In the third step (S3), a decompression step (first pressureequalization step), an adsorption step (first adsorption step) and apressurization step (second pressure equalization step) are performed inthe adsorption tower A, B, and C, respectively. Specifically, as shownin FIG. 10C, remaining gas discharged from the adsorption tower A isintroduced into the adsorption tower C through the automatic valve h,the remaining gas outlet pipe 3, the flow rate controlling valve 7, theremaining gas inlet pipe 4 and the automatic valve m. As a result, thedecompression for the adsorption tower A and the pressurization for theadsorption tower C are performed at the same time. Material gas isintroduced into the adsorption tower B through the material gas pipe 1and the automatic valve c. The adsorbent loaded in the adsorption towerB removes unnecessary gas components from the material gas for providingproduct gas. The product gas is discharged from the adsorption tower Band then collected through the automatic valve 1 and the product gaspipe 2.

[0013] In the fourth through the sixth steps (S4-S6), process stepsdescribed below are performed in each of the adsorption towers. In theadsorption tower A, a desorption step, a cleaning step and apressurization step (second pressure equalization step) are performed.These process steps are similar to those performed in the adsorptiontower C in the first through the third steps. In the adsorption tower B,an adsorption step (second adsorption step), an adsorption step (thirdadsorption step) and a decompression step (first pressure equalizationstep) are performed. These process steps are similar to those performedin the adsorption tower A in the first through the third steps. In theadsorption tower C, a pressurization step (first pressurization step), apressurization step (second pressurization step) and an adsorption step(first adsorption step) are performed. These process steps are similarthose performed in the adsorption tower B in the first through the thirdsteps.

[0014] In the seventh through the ninth steps, the process stepsdescribed below are performed in each of the adsorption towers. In theadsorption tower A, a pressurization step (first pressurization step), apressurization step (second pressurization step) and an adsorption step(first adsorption step) are performed. These process steps are similarto those performed in the adsorption tower B in the first through thethird steps. In the adsorption tower B, a desorption step, a cleaningstep and a pressurization step (second pressure equalization step) areperformed. These process steps are similar to those performed in theadsorption tower C in the first through the third steps. In theadsorption tower C, an adsorption step (second adsorption step), anadsorption step (third adsorption step) and a decompression step (firstpressure equalization step) are performed. These process steps aresimilar to those performed in the adsorption tower A in the firstthrough the third steps.

[0015] By repetitively performing the above-described first through ninesteps in each of the adsorption towers A-C, unnecessary gas componentsare removed from the material gas, thereby providing product gascontaining a high concentration of hydrogen.

[0016] As described above, in the prior art PSA process, product gas isintroduced from an adsorption tower (e.g. the adsorption tower A in thesecond step) in which adsorption is being performed to anotheradsorption tower (the adsorption tower C in the second step) in whichdesorption is finished for cleaning this adsorption tower. To avoidwasteful discharging of product gas, the amount of product gasintroduced is set to be smaller than the volume of the loaded adsorbent.Further, in the prior art process, for efficiently utilizinghigh-pressure gas in an adsorption tower, internal pressure equalizationis performed between an adsorption tower in which adsorption is finished(e.g. the adsorption tower A in the third step) and an adsorption towerin which adsorption is to be performed (the adsorption tower C in thethird step).

[0017] Various improvements have been proposed also for adsorbents forthe PSA process. For example, JP-A-10(1998)-212103 discloses zeolitehaving high adsorptivity for nitrogen gas and carbon monoxide gas forremoving these gas components from material gas. The zeolite has afaujasite structure with a Si/Al ratio lying in the range of 1 to 3 andwith a lithium-exchange ratio of no less than 70%.

[0018] As described above, the prior art PSA process has been improvedin various ways. However, in spite of such improvements, theconventional PSA process still has the following problems to be solved.

[0019] The first problem relates to the yield of target gas.Conventionally, as described above, each of adsorption towers is cleanedusing product gas for the purpose of enhancing the regenerationefficiency of the adsorbent and the yield of target gas. Actually,however, it is found that the yield is not increased as much asexpected.

[0020] The second problem is caused by the pressure equalization stepbetween two adsorption towers. As described above, by introducingremaining gas from an adsorption tower on the high pressure side to anadsorption tower on the low pressure side, target gas included inremaining gas can be collected. However, the remaining gas contains notonly the target gas but also unnecessary gas components. Therefore, partof the unnecessary gas components adsorbs to the adsorbent in theadsorption tower to which the remaining gas is introduced, so that theadsorbent cannot exhibit as much adsorption effect as expected.

[0021] The third problem is an increase in size of the apparatus due tothe use of a plurality of adsorbents. As the material gas for the PSAprocess, use may be made of mixed gas obtained by steam-reforminghydrocarbon (methanol or natural gas), for example. For example, in thecase of reforming methanol, the composition of the mixed gas is about75% hydrogen gas, about 24% carbon dioxide gas and about 1% carbonmonoxide gas. To obtain high purity hydrogen gas (target gas) from suchmixed gas, both of carbon dioxide component and carbon monoxidecomponent need be removed. As described above, in the prior art PSAprocess, zeolite is used as the adsorbent for removing carbon monoxidecomponent, whereas activated carbon-based adsorbent is used for removingcarbon dioxide component. Therefore, to remove both carbon dioxidecomponent and carbon monoxide component, the two kinds of adsorbentsneed be loaded in each of the adsorption towers. To load the pluralkinds of adsorbents, adsorption towers of large capacity need be used,which disadvantageously increase the size of the entire separationapparatus.

[0022] The reason why two kinds of adsorbents are needed for removingcarbon dioxide and carbon monoxide is as follows.

[0023] As described above, the PSA process is a gas separation methodwhich utilizes the fact that the amount of an unnecessary gas componentadsorbed varies in accordance with the pressure variation in theadsorption tower. Therefore, to effectively remove an unnecessary gascomponent in the PSA process, a condition need be satisfied that theunnecessary gas component is likely to be adsorbed by the adsorbentunder high pressure while it is unlikely to be adsorbed (i.e. is likelyto be desorbed) under low pressure. However, when a prior artzeolite-based adsorbent is used for carbon dioxide, this condition isnot satisfied. This point will be described below in detail withreference to FIG. 17.

[0024] The graph in FIG. 17 shows how adsorption isotherms (25° C.) forcarbon dioxide gas become when three kinds of adsorbents (a 85%Li-exchanged zeolite, a Ca-exchanged A-type zeolite and a carbon-basedadsorbent) are used. The signs “Li85Z”, “CaAZ” and “Car.” in the figureindicate the 85% Li-exchange zeolite, the Ca-exchange A-type zeolite andthe carbon-based adsorbent, respectively. The 85% Li-exchange zeolitehas a faujasite structure, a Si/Al ratio of 1 and a lithium-exchangeratio of 85%. In the graph of FIG. 17, the abscissa is adsorptionequilibrium pressure (A.E.P.), whereas the ordinate is adsorbed amountof carbon dioxide (CO₂ Ad.) The following is understood from the graph.As the equilibrium adsorption pressure increases, the amount of carbondioxide adsorbed by the carbon-based adsorbent increases generallylinearly. On the other hand, in the case of 85% Li-exchange zeolite andCa-exchange A-type zeolite, the adsorbed amount of carbon dioxiderapidly increases initially but becomes generally constant when acertain value is exceeded. Specifically, in the case of 85% Li-exchangezeolite, the increasing rate of the adsorbed amount becomes small fromwhen the equilibrium adsorption pressure exceeds approximately 1000Torr. In the case of Ca-exchange A-type zeolite, the increasing rate ofthe adsorbed amount becomes small from when the equilibrium adsorptionpressure exceeds approximately 750 Torr.

[0025] From the above, it is understood that the 85% Li-exchange zeoliteand the Ca-exchange A-type zeolite are not suitable for removing carbondioxide component in the PSA process, although the carbon-basedadsorbent is effective for the removal. This point will be describedusing, as an example, a mixed gas containing about 75% hydrogen gas,about 24% carbon dioxide gas and about 1% carbon monoxide gas. Forexample, when the adsorption pressure for the mixed gas is set to 0.8MPa and the desorption pressure for the gas is set to ⅛ (approximatelyequal to atmospheric pressure) of the adsorption pressure, the partialpressure of carbon dioxide gas contained in the mixed gas becomes about0.192 MPa (1440 Torr) during adsorption and about 0.024 MPa (180 Torr)during desorption. As will be understood from the graph of FIG. 17, inthe case where the carbon-based adsorbent is used, the adsorbed amountis 66 ml/g when the partial pressure of carbon dioxide gas is 1440 Torrwhereas the adsorption amount is 18 ml/g when the partial pressure is180 Torr. This indicates that 48(=66−18) ml/g of carbon dioxide gas isremoved by varying the partial pressure of carbon dioxide gas in therange of 180 to 1440 Torr.

[0026] In the case where the Ca-exchange A-type zeolite is used, theadsorbed amount is 85 ml/g when the partial pressure of carbon dioxidegas is 1440 Torr whereas the adsorbed amount is 48 ml/g when the partialpressure is 180 Torr. Therefore, the amount of carbon dioxide gasremoved is 37(=85−48) ml/g. In the case where the 85% Li-exchangezeolite is used, the adsorbed amount is 119 ml/g when the partialpressure of carbon dioxide gas is 1440 Torr whereas the adsorbed amountis 82 ml/g when the partial pressure is 180 Torr. Therefore, the amountof carbon dioxide gas removed is 37(=119−82) ml/g.

[0027] In this way, a larger amount of carbon dioxide gas can be removedby the carbon-based adsorbent than by the zeolite-based adsorbent.Conventionally, therefore, a zeolite-based adsorbent has been consideredto be unsuitable for the removal of carbon dioxide component in the PSAprocess.

DISCLOSURE OF THE INVENTION

[0028] The present invention is conceived under the circumstancesdescribed above. Therefore, an object of the present invention is toenhance the yield of target gas by improving the steps performed in thePSA process.

[0029] Another object of the present invention is to provide anadsorbent capable of removing both a carbon dioxide component and acarbon monoxide component.

[0030] According to a first aspect of the present invention, there isprovided a method for separating hydrogen gas from material gas. Thismethod utilizes a plurality of adsorption towers each of which is loadedwith an adsorbent and is provided with a product gas outlet. Accordingto the method, one cycle comprising an adsorption step, a decompressionstep, a desorption step, a cleaning step and a pressurization step isrepetitively performed. Specifically, in the adsorption step, anunnecessary gas component contained in the material gas is adsorbed bythe adsorbent for outputting hydrogen-rich product gas through theproduct gas outlet. In the decompression step, pressure in an adsorptiontower is reduced. In the desorption step, the unnecessary component isdesorbed from the adsorbent. In the cleaning step, the adsorption toweris cleaned by introducing cleaning gas into the adsorption tower. In thepressurizing step, pressure in the adsorption tower is raised.

[0031] The decompression step includes introducing gas remaining in theadsorption tower into a selected adsorption tower as cleaning gas. Theremaining gas is introduced in an amount 2 to 7 times the volume of theadsorbent loaded in the selected adsorption tower as converted intovolume at common temperature and under atmospheric pressure.

[0032] As described above, product gas is utilized as cleaning gas inthe prior art method. When remaining gas is introduced as cleaning gasin an amount 2 to 7 times the volume of the loaded adsorbent as is inthe present invention, target gas can be efficiently recovered.

[0033] Preferably, the cleaning step includes an additional cleaningstep performed by introducing product gas obtained from an adsorptiontower undergoing an adsorption step as cleaning gas. The target gas canbe recovered more efficiently by performing such an additional cleaningwith product gas after the above-described cleaning with remaining gas.

[0034] Preferably, the cleaning step in the one cycle includes a firstcleaning step and a second cleaning step performed after the firstcleaning step, and the decompression step in the one cycle includes afirst decompression step and a second decompression step performed afterthe first decompression step. The first and the second decompressionsteps are performed by discharging remaining gas through the product gasoutlet.

[0035] Preferably, the first cleaning step in a first adsorption toweris performed by introducing therein remaining gas discharged from asecond adsorption tower during the second decompression step through theproduct gas outlet of the first adsorption tower as cleaning gas, andthe second cleaning step in the first adsorption tower is performed byintroducing therein remaining gas discharged from a third adsorptiontower during the first decompression step through the product gas outletof the first adsorption tower as cleaning gas.

[0036] Preferably, in addition to the first and the second cleaningsteps, a third cleaning step is performed by introducing product gasobtained from an adsorption tower undergoing an adsorption step ascleaning gas.

[0037] Preferably, in the one cycle, the decompression step, thedesorption step, the first cleaning step, the desorption step, thesecond cleaning step and the third cleaning step are performed in thementioned order in each of the adsorption towers.

[0038] Preferably, the maximum pressure in the adsorption step lies in arange of 0.2 to 3.6 MPa (absolute pressure), whereas the minimumpressure in the desorption step lies in a range of atmospheric pressureto 0.15 MPa (absolute pressure).

[0039] The material gas may contain carbon dioxide gas as theunnecessary gas component, for example.

[0040] According to a second aspect of the present invention, there isprovided a method for removing at least carbon dioxide gas from materialgas to obtain target gas. The method includes an adsorption step and adesorption step. In the adsorption step, the material gas is introducedinto an adsorption tower loaded with an adsorbent for removing anunnecessary gas component including carbon dioxide by the adsorbent. Inthe desorption step, pressure in the adsorption tower is reduced toseparate the unnecessary gas component from the adsorbent. Theadsorption step and the desorption step constitute one cycle, which isrepetitively performed. The minimum pressure in the desorption step isset to be approximately equal to atmospheric pressure. As the adsorbent,use may be made of zeolite having a faujasite structure with a Si/Alratio lying in a range of 1 to 1.5 and a lithium-exchange ratio of noless than 95%.

[0041] Preferably, the maximum pressure in the adsorption step lies in arange of 0.5 to 4 MPa (absolute pressure).

[0042] Preferably, the material gas is a gas obtained by steam-reforminga hydrocarbon-based compound and contains carbon dioxide gas andhydrogen gas.

[0043] Further, the gas obtained by steam-reforming contains carbonmonoxide, and the material gas may be that obtained after the content ofthe carbon monoxide is reduced by conversion.

[0044] According to a third aspect of the present invention, there isprovided a gas separation apparatus comprising at least one adsorptiontower and an adsorbent loaded in the adsorption tower. Material gascontaining carbon dioxide gas is introduced into the adsorption tower.The adsorbent removes unnecessary gas (including carbon dioxide) fromthe material gas. As the adsorbent, use may be made of zeolite having afaujasite structure with a Si/Al ratio lying in a range of 1 to 1.5 anda lithium-exchange ratio of no less than 95%.

BRIEF DESCRIPTION OF THE DRAWINGS

[0045]FIG. 1 schematically illustrates a PSA separation apparatusincluding three adsorption towers.

[0046]FIG. 2 is a table showing process steps performed in a separationmethod according to a first embodiment of the present invention.

[0047] FIGS. 3A-3F illustrate gas flows in performing the separationmethod of the first embodiment.

[0048]FIG. 4 is a table showing process steps performed in a separationmethod according to a second embodiment of the present invention.

[0049] FIGS. 5A-5C illustrate gas flows in additional process steps inthe separation method of the second embodiment.

[0050]FIG. 6 schematically illustrates a PSA separation apparatus usedfor performing a separation method according to a third embodiment ofthe present invention.

[0051]FIG. 7 is a table showing process steps performed in theseparation method of the third embodiment.

[0052] FIGS. 8A-8L illustrate gas flows in performing the separationmethod of the third embodiment.

[0053]FIG. 9 is a table showing process steps performed in a prior artseparation method.

[0054] FIGS. 10A-10I illustrate gas flows in performing the separationmethod of FIG. 9.

[0055]FIG. 11 schematically illustrates a separation apparatus which canbe used for performing a gas separation method according to the presentinvention.

[0056] FIGS. 12A-12I illustrate gas flows in the separation apparatus ofFIG. 11.

[0057]FIG. 13 is a table showing process steps performed in theapparatus of FIG. 11.

[0058]FIG. 14 is a graph showing the effectiveness of a zeolite-basedadsorbent according to the present invention.

[0059]FIGS. 15 and 16 illustrate adsorption characteristics of azeolite-based adsorbent according to the present invention and adsorbentcharacteristics of prior art zeolite-based adsorbents.

[0060]FIG. 17 is a graph showing adsorption characteristics of prior artadsorbents for carbon dioxide.

BEST MODE FOR CARRYING OUT THE INVENTION

[0061] Preferred embodiments of the present invention will be describedbelow in detail with reference to the accompanying drawings.

[0062]FIG. 1 illustrates a PSA separation apparatus X used for embodyinga hydrogen gas separation method according to a first embodiment of thepresent invention. The PSA separation apparatus X includes threeadsorption towers A, B and C. A material gas is introduced into each ofthe adsorption towers A-C. As the material gas, use may be used of gascontaining about 75% hydrogen gas, about 20% carbon dioxide gas andabout 1% carbon monoxide gas by volume ratio, for example. (The materialgas further contains nitrogen gas and methane gas, for example.) Each ofthe adsorption towers A-C is loaded with an adsorbent for removing anunnecessary gas component from the material gas.

[0063] As will be understood from the description given below, thehydrogen gas separation method according to the first embodiment makesit possible to recover a higher yield of target gas (hydrogen gas) thana prior art method by innovating the process steps in the PSA process.Therefore, an adsorbent of the kind used in the prior art method may beused for loading in each of the adsorption towers A-C. Specifically, toremove only carbon dioxide and methane gas from the material gas, anactivated carbon-based adsorbent may be used. To remove carbon monoxideand nitrogen gas only, a zeolite-based adsorbent may be used. To removewater vapor, alumina may be used, for example. To remove both carbondioxide and carbon monoxide from the material gas, both of activatedcarbon-based adsorbent and zeolite may be used. In this case, however,the size of the entire separation apparatus becomes large as pointed outas a problem of the prior art method. This problem can be avoided byusing zeolite having a novel structure provided according to the presentinvention (which will be described later.)

[0064] The PSA separation apparatus X further includes a material gaspipe 1, a product gas pipe 2, a remaining gas outlet pipe 3, a remaininggas inlet pipe 4, a product purge pipe 5 and a discharge pipe 6. In FIG.1, the sign “Gmat” indicates material gas, whereas the sign “Gpro”indicates product gas. The pipes 1-6 are provided with automatic valvesa-p. The remaining gas outlet pipe 3 and the product purge pipe 5 areprovided with flow rate controlling valves 7 and 8, respectively.

[0065] An adsorption step, a decompression step, a desorption step, acleaning step and a pressurization step are performed in each of theadsorption towers A-C by selectively opening or closing the automaticvalves a-p. Specifically, as shown in FIG. 2, six steps (S1-S6) areperformed in each of the adsorption towers A-C. In this figure, the sign“Ad.” indicates an adsorption step, the sign “Decomp.” indicates adecompression step, the sign “Desorp.” indicates a desorption step, thesign “Cl.” indicates a cleaning step, and the sign “Pres.” indicates apressurization step. For example, in the first step (S1), a firstadsorption step (1st Ad.) is performed in the adsorption tower A, acleaning step (Cl.) is performed in the adsorption tower B, and adecompression step (Decomp.) is performed in the adsorption tower C. Atthat time, the automatic valves a(Va), d(Vd), i(Vi), j(Vj) and n(Vn) areopen (o), whereas other automatic valves are closed (x).

[0066] FIGS. 3A-3F illustrate gas flow in the separation apparatus X inthe first through the sixth steps. The illustrated six steps constituteone cycle (hereinafter referred to as “current cycle”) which theseparation apparatus X performs in the steady operation. The currentcycle is performed following the immediately preceding cycle(hereinafter referred to as “previous cycle”). That is, the first stepshown in FIG. 3A is performed after the sixth step of the previouscycle.

[0067] As described above, in the first step (S1), a first adsorptionstep, a cleaning step and a decompression step are performed in theadsorption towers A, B and C, respectively. The gas flow in this stateis shown in FIG. 3A. Specifically, material gas (Gmat) is introducedinto the adsorption tower A through the material gas pipe 1 and theautomatic valve a. In the adsorption tower A, an unnecessary gascomponent is removed by the adsorbent and product gas (Gpro) isdischarged. The product gas is collected through the automatic valve iand the product gas pipe 2.

[0068] Remaining gas (Grem) discharged from the adsorption tower C isintroduced into the adsorption tower B through the automatic valve n,the remaining gas outlet pipe 3, the flow rate controlling valve 7, theremaining gas inlet pipe 4 and the automatic valve j. In the sixth stepof the previous cycle, adsorption has been performed in the adsorptiontower C whereas desorption has been performed in the adsorption tower B(See FIG. 3F). Therefore, the internal pressure of the adsorption towerC is higher than that of the adsorption tower B. Since gas is introducedfrom the adsorption tower C into the adsorption tower B in this state,the pressure in the adsorption tower C drops. On the other hand,remaining gas (Grem) is discharged from the adsorption tower B. Thisremaining gas is discharged out of the separation apparatus X throughthe automatic valve d and the discharge pipe 6.

[0069] The amount of remaining gas (cleaning gas) introduced from theadsorption tower C into the adsorption tower B is controlled y the flowrate controlling valve 7. According to the present invention, the amountof gas introduced into the adsorption tower B is 2 to 7 times the volumeof the adsorbent loaded in the adsorption tower B, as converted intovolume at common temperature and under atmospheric pressure. With thissetting, at least part of the remaining gas introduced into theadsorption tower B is discharged from the adsorption tower B.

[0070] As described above, in the sixth step (S6) of the previous cycle,an adsorption step has been performed in the adsorption tower C whereasa desorption step has been performed in the adsorption tower B.Therefore, the concentration of the unnecessary gas component in thecleaning gas introduced to the adsorption tower B is lower than that inthe gas remaining in the adsorption tower B. Therefore, during thecleaning step, the concentration of the unnecessary gas component(partial pressure of the unnecessary gas) in the adsorption tower Bdecreases in comparison with that before the cleaning step, whichpromotes the desorption of the unnecessary gas component from theadsorbent. Further, since the cleaning gas is introduced into theadsorption tower B by an amount larger than that in the prior artmethod, the unnecessary gas component in the adsorption tower B isreliably discharged. As a result, the concentration of the unnecessarygas component in the adsorption tower B is low when the cleaning step isfinished, so that the adsorbent is duly regenerated. In the illustratedembodiment, the maximum pressure reached in the adsorption tower A(adsorption step) lies in the range of 0.2 to 3.6 MPa (absolutepressure, which holds true hereinafter).

[0071] In the second step (S2) shown in FIG. 3B, a second adsorptionstep, a pressurization step and a desorption step are performed in theadsorption tower A, B and C, respectively (See FIG. 2). Similarly to thefirst step, material gas is introduced into the adsorption tower A whileproduct gas is discharged from the tower. The product gas is partiallycollected and partially introduced into the adsorption tower B throughthe product purge pipe 5, the automatic valve p, the flow ratecontrolling valve 8, the remaining gas inlet pipe 4 and the automaticvalve j. As a result, the pressure in the adsorption tower B is raised.On the other hand, the internal pressure of the adsorption tower C hasbeen reduced through the first step (S1). Further, the automatic valvese, m, n and o are closed whereas the automatic valve f is open.Therefore, the unnecessary gas component is desorbed from the adsorbentin the adsorption tower C and discharged from the adsorption tower C.The desorbed gas (Gdes) is collected through the discharge pipe 6 andthe automatic valve f. The minimum pressure in the adsorption tower C inthe desorption step lies in the range of atmospheric pressure to 0.15MPa.

[0072] In the second step, the pressurization of the adsorption tower Bis performed by the introduction of product gas. Therefore, an undulylarge amount of unnecessary gas component is not introduced into theadsorption tower B. Further, the adsorption tower B has been cleanedwith the remaining gas discharged from the adsorption tower C in thefirst step (S1). Also for this reason, after the second step isfinished, the concentration of the unnecessary gas component in theadsorption tower B is low. It is to be noted that, in the first step,the adsorption tower C is in the state immediately after an adsorptionstep is finished. At that time, therefore, the concentration of theunnecessary gas component in the remaining gas in the adsorption tower Cis sufficiently low. Therefore, the use of this remaining gas for thecleaning of the adsorption tower B causes no problem.

[0073] In the third step shown in FIG. 3C, a decompression step, a firstadsorption step and a cleaning step are performed in the adsorptiontowers A, B and C, respectively. Further, in the fourth step shown inFIG. 3D, a desorption step, a second adsorption step and apressurization step are performed in the adsorption towers A, B and C,respectively. The decompression step and the desorption step performedin the adsorption tower A in these steps are performed similarly tothose performed in the adsorption tower C in the first and second steps.The first adsorption step and the second adsorption step in theadsorption tower B are performed similarly to those performed in theadsorption tower A in the first and second steps. The cleaning step andthe pressurization step in the adsorption tower C are performedsimilarly to those performed in the adsorption tower B in the first andsecond steps.

[0074] As described above, the concentration of the unnecessary gascomponent in the adsorption tower B is low after the second step.Therefore, the adsorption step in the adsorption tower B (the third andthe fourth steps) can be performed under the condition where theconcentration of the unnecessary gas component is low. As a result, theconcentration of the unnecessary gas component is extremely low in theproduct gas obtained from the adsorption tower B. In this way, theseparation method of this embodiment is capable of providing extremelyhigh purity product gas.

[0075] In the fifth step shown in FIG. 3E, a cleaning step, adecompression step and a first adsorption step are performed in theadsorption towers A, B and C, respectively. Further, in the sixth stepshown in FIG. 3F, a pressurization step, a desorption step and a secondadsorption step are performed in the adsorption towers A, B and C,respectively. The cleaning step and the pressurization step of theadsorption tower A in these steps are performed similarly to thoseperformed in the adsorption tower B in the first and the second steps.The decompression step and the desorption step in the adsorption tower Bare performed similarly to those performed in the adsorption tower C inthe first and the second steps. The first adsorption step and the secondadsorption step in the adsorption tower C are performed similarly tothose performed in the adsorption tower A in the first and the secondsteps.

[0076] By repetitively performing the above-described six process steps(S1-S6), the unnecessary gas component is removed from the material gas,thereby providing product gas with a high hydrogen gas concentration.

[0077] Next, a hydrogen gas separation method according to a secondembodiment of the present invention will be described with reference toFIGS. 4 and 5A-5C. In addition to the steps (first through sixth steps)of the separation method of the first embodiment, the separation methodaccording to the second embodiment includes cleaning steps (AS1′, AS3′and AS5′) which utilize product gas. As shown in FIG. 4, an additionalcleaning step (2nd Cl.) of the adsorption tower B is performed in stepAS1′ between steps S1 and S2. An additional cleaning step of theadsorption tower C is performed in step AS3′ between steps S3 and S4. Anadditional cleaning of the adsorption tower A is performed in step AS5′between steps S5 and S6. The sign “3rd Ad.” in FIG. 4 indicates a thirdadsorption step.

[0078]FIGS. 5A, 5B and 5C indicate gas flow in the steps AS1′, AS3′ andAS5′, respectively.

[0079] In the step AS1′ (FIG. 5A), material gas is introduced into theadsorption tower A while product gas is discharged from the tower in amanner similar to the second step of the first embodiment. The productgas is partially collected and partially introduced into the adsorptiontower B through the product purge pipe 5, the automatic valve p, theflow rate controlling valve 8, the remaining gas inlet pipe 4 and theautomatic valve j (FIG. 1). By the flow rate controlling valve 8, theamount of product gas (cleaning gas) introduced into the adsorptiontower B is controlled to be 0.1 to 1 time the volume of the adsorbentloaded in the adsorption tower B. As a result of the introduction of thecleaning gas, remaining gas is discharged from the adsorption tower Bthrough the automatic valve d and the discharge pipe 6. In theadsorption tower C, a first desorption step is performed similarly tothe second step of the first embodiment.

[0080] In the step AS3′ (FIG. 5B), a first desorption step is performedin the adsorption tower A similarly to that performed in the adsorptiontower C in the step AS1′. Further, a second adsorption step is performedin the adsorption tower B similarly to that performed in the adsorptiontower A in the step AS1′. Further, a second cleaning step is performedin the adsorption tower C similarly to that performed in the adsorptiontower B in the step AS1′.

[0081] In the step AS5′ (FIG. 5C), an additional (second) cleaning isperformed in the adsorption tower A similarly to that performed in theadsorption tower B in the step AS1′. Further, a first desorption step isperformed in the adsorption tower B similarly to that performed in theadsorption tower C in the step AS1′. Further, a second adsorption stepis performed in the adsorption tower C similarly to that performed inthe adsorption tower A in the step AS1′.

[0082] According to the second embodiment of the present invention, asecond cleaning step is performed after a first cleaning step. The firstcleaning step is performed utilizing remaining gas obtained from anadsorption tower in which adsorption has been finished. The secondcleaning step is performed utilizing product gas obtained from anadsorption tower in which adsorption is proceeding. Specifically, usingthe adsorption tower B as an example, the first cleaning step (FIG. 3A)is performed utilizing remaining gas obtained from the adsorption towerC in which adsorption has been finished, whereas the second cleaningstep (FIG. 5A) is performed utilizing product gas obtained from theadsorption tower A in which adsorption is proceeding. Herein, it is tobe noted that the concentration of the unnecessary gas component in theproduct gas is lower than that in the remaining gas. Therefore, theinterior of each adsorption tower can be cleaned more reliably byfirstly cleaning with remaining gas and then cleaning with product gas,which enhances the regeneration efficiency of the adsorbent. Further,owing to the previous cleaning with the remaining gas, the amount ofproduct gas required for the second cleaning is reduced. Therefore, itis possible to prevent or inhibit the product gas introduced into theadsorption tower from being discharged, which enhances the recovery ofhydrogen gas.

[0083] Next, a hydrogen gas separation method according to a thirdembodiment of the present invention will be described with reference toFIGS. 6, 7 and 8A-8L.

[0084]FIG. 6 illustrates main structural members of a PSA separationapparatus X′ for embodying the separation method of the thirdembodiment. The separation apparatus X′ includes four adsorption towersA′, B′, C′ and D′. The reference signs a′-u′ indicate automatic valves.The separation apparatus X′ further includes a material gas pipe 1′, aproduct gas pipe 2′, a remaining gas outlet pipe 3′, a remaining gasinlet pipe 4′, a product purge pipe 5′ and a discharge pipe 6′. Theremaining gas outlet pipe 3′ and the remaining gas inlet pipe 4′ areprovided with flow rate controlling valves 7′ and 8′, respectively.

[0085] As will be understood from FIG. 7, in the separation method ofthe third embodiment, one cycle consisting of twelve steps (a first stepS1 through a twelfth step 12) are repetitively performed in each of theadsorption towers. FIGS. 8A-8L illustrate gas flow in the first step S1through the twelfth step S12.

[0086] In the first step (S1), a first adsorption step (1st Ad.), asecond cleaning step (2nd Cl.), a second desorption step (2nd Desorp.)and a first decompression step (1st Decomp.) are performed in theadsorption towers A′, B′, C′ and D′, respectively (FIG. 7). At thattime, material gas (Gmat) is introduced into the adsorption tower A′through the material gas pipe 1′ and the automatic valve a′ (FIG. 8A).An unnecessary gas component is removed by the adsorbent in theadsorption tower A′ and product gas (Gpro) is discharged to the outsideof the tower. The product gas is collected through the automatic valvek′ and the product gas pipe 2′.

[0087] Remaining gas (cleaning gas) discharged from the adsorption towerD′ is introduced into the adsorption tower B′ through the automaticvalve s′, the remaining gas outlet pipe 3′, the flow rate controllingvalve 7′, the remaining gas inlet pipe 4′ and the automatic valve 1′.The adsorption tower D′ has previously undergone an adsorption stepwhereas the adsorption tower B′ has previously undergone a (third)desorption step (twelfth step S12). Therefore, the internal pressure ofthe adsorption tower D′ is higher than that of the adsorption tower B′.Therefore, when the remaining gas of the adsorption tower D′ isintroduced into the adsorption tower B′, the internal pressure of theadsorption tower D′ drops while remaining gas is discharged from theadsorption tower B′ through the automatic valve d′ and the dischargepipe 6′.

[0088] The amount of remaining gas introduced from the adsorption towerD′ into the adsorption tower B′ is controlled by the flow ratecontrolling valve 7′. According to the present invention, the amount ofgas introduced is set to be 2 to 5 times the volume of the adsorbentloaded in the adsorption tower B′ (as converted into volume at commontemperature and under atmospheric pressure).

[0089] In the adsorption tower C′, a desorption step is continuedfollowing the twelfth step (S12). In the first step (S1), the internalpressure of the adsorption tower C′ has been reduced, and the automaticvalves e′, o′, p′ and q′ are closed whereas the automatic valve f′ isopen. Therefore, the unnecessary gas component is desorbed from theadsorbent in the adsorption tower C′ and discharged together with thegas in the adsorption tower C′. The discharged gas is collected throughthe automatic valve f′ and the discharge pipe 6′.

[0090] In the second step (S2), a second adsorption step (2nd Ad.), anidling step (Id), a first cleaning step (1st Cl.), a seconddecompression step (2nd Decomp.) are performed in the adsorption towersA′, B′, C′ and D′, respectively. The gas flow at that time isillustrated in FIG. 8B. As shown in the figure, the adsorption step inthe adsorption tower A′ is performed similarly to that in the firststep. The adsorption tower B′ is kept idling (Id) with the automaticvalves c′, d′, l′, m′ and n′ closed. Remaining gas (cleaning gas)discharged from the adsorption tower D′ is introduced into theadsorption tower C′ through the automatic valve s′, the remaining gasoutlet pipe 3′, the flow rate controlling valve 7′, the remaining gasinlet pipe 4′ and the automatic valve o′. As a result, the internalpressure of the adsorption tower D′ drops while remaining gas isdischarged from the adsorption tower C′ through the automatic valve f′and the discharge pipe 6′. The amount of remaining gas introduced fromthe adsorption tower D′ into the adsorption tower C′ is controlled bythe flow rate controlling valve 7′. According to the present invention,the amount of gas introduced is set to be 1 to 3 times the volume of theadsorbent loaded in the adsorption tower C′ (as converted into volume atcommon temperature and under atmospheric pressure).

[0091] In the third step (S3), a third adsorption step (3rd Ad.), apressurization step (Pres.), a third desorption step (3rd Desorp.) and afirst desorption step (1st Desorp.) are performed in the adsorptiontowers A′, B′, C′ and D′, respectively. The gas flow at that time isillustrated in FIG. 8C.

[0092] As will be understood from FIG. 8C, material gas is introducedinto the adsorption tower A′ while product gas is discharged from thetower similarly to the first step. The product gas is partiallycollected and partially introduced into the adsorption tower B′ throughthe product purge pipe 5′, the automatic valve u′, the flow ratecontrolling valve 8′, the remaining gas inlet pipe 4′ and the automaticvalve 1′. As a result, the pressure in the adsorption tower B′increases.

[0093] In the adsorption tower C′, the unnecessary gas component isdesorbed from the adsorbent similarly to the first step. The desorbedgas is discharged through the automatic valve f′ and the discharge pipe6′. Similarly, desorption of the unnecessary gas component occurs alsoin the adsorption tower D′. At that time, the automatic valves g′, r′,s′ and t′ are closed while the automatic valve h′ is open. The desorbedgas is discharged together with the remaining gas of the adsorptiontower D′ through the automatic valve h′ and the discharge pipe 6′.

[0094] In respective fourth, fifth and sixth steps (S4, S5 and S6), afirst decompression step (1st Decomp.), a second decompression step (2ndDecomp.) and a first desorption step (1st Desorp.) are performed in theadsorption tower A′, similarly to those performed in the adsorptiontower D′ in the steps S1-S3. Further, in the adsorption tower B′, afirst adsorption step (1st Ad.), a second adsorption step (2nd Ad.) anda third adsorption step (3rd Ad.) are performed similarly to thoseperformed in the adsorption tower A′ in the steps S1-S3. In theadsorption tower C′, a second cleaning step (2nd Cl.), an idling step(Id) and a pressurization step (Pres.) are performed similarly to thoseperformed in the adsorption tower B′ in the steps S1-S3. In theadsorption tower D′, a second desorption step (2nd Desorp.), a firstcleaning step (1st Cl.) and a third desorption step (3rd Desorp.) areperformed similarly to those performed in the adsorption tower C′ in thesteps S1-S3.

[0095] In respective seventh, eighth and ninth steps (S7, S8 and S9), asecond desorption step (2nd Desorp.), a first cleaning step (1st Cl.)and a third desorption step (3rd Desorp.) are performed in theadsorption tower A′, similarly to those performed in the adsorptiontower C′ in the steps S1-S3. In the adsorption tower B′, a firstdecompression step (1st Decomp.) a second decompression step (2ndDecomp.) and a first desorption step (1st Desorp.) are performedsimilarly to those performed in the adsorption tower D′ in the stepsS1-S3. In the adsorption tower C′, a first adsorption step (1st Ad.), asecond adsorption step (2nd Ad.) and a third adsorption step (3rd Ad.)are performed similarly to those performed in the adsorption tower A′ inthe steps S1-S3. In the adsorption tower D′, a second cleaning step (2ndCl.), an idling step (Id) and a pressurization step (Pres.) areperformed similarly to those performed in the adsorption tower B′ in thesteps S1-S3.

[0096] In respective tenth, eleventh and twelfth steps (S10, S11 andS12), a second cleaning step (2nd Cl.), an idling step (Id.) and apressurization step (Pres.) are performed in the adsorption tower A′,similarly to those performed in the adsorption tower B′ in the stepsS1-S3. In the adsorption tower B′, a second desorption step (2ndDesorp.), a first cleaning step (1st Cl.) and a third desorption step(3rd Desorp.) are performed similarly to those performed in theadsorption tower C′ in the steps S1-S3. In the adsorption tower C′, afirst decompression step (1st Decomp.), a second decompression step (2ndDecomp.) and a first desorption step (1st Desorp.) are performedsimilarly to those performed in the adsorption tower D′ in the stepsS1-S3. In the adsorption tower D′, a first adsorption step (1st Ad.), asecond adsorption step (2nd Ad.) and a third adsorption step (3rd Ad.)are performed similarly to those performed in the adsorption tower A′ inthe steps S1-S3.

[0097] In all of the adsorption towers A′, B′, C′ and D′, the maximumpressure in the first through the third adsorption steps is set to liein the range of 0.2 to 3.6 Mpa, whereas the minimum pressure in thedesorption steps is set to lie in the range of atmospheric pressure to0.15 MPa.

[0098] In the above-described separation method, from the adsorptiontower D′ in which the adsorption steps (S10-S12) have been finished,remaining gas (cleaning gas) is introduced in the first decompressionstep (S1) into the adsorption tower B′ (FIG. 8A). At that time, theadsorption tower B′ is undergoing the second cleaning step. Further inthe second decompression step (S2), remaining gas (cleaning gas) isintroduced from the adsorption tower D′ to the adsorption tower C′ (FIG.8B). At that time, the adsorption tower C′ is undergoing the firstcleaning step. As shown in FIGS. 8A and 8B, remaining gas is taken outthrough the upper side of the adsorption tower D′ (i.e. from the sidethrough which product gas is discharged in step S12). Therefore, theremaining gas contains a lower concentration of unnecessary gascomponent than in the case where the remaining gas is discharged throughthe lower side of the adsorption tower D′. Further, the remaining gastaken out in the first step (S1) contains a lower concentration ofunnecessary gas component than the remaining gas taken out in the secondstep (S2). Thus, according to the separation method, the adsorptiontower C′ in the first cleaning step is cleaned with remaining gascontaining a relatively high concentration of unnecessary gas component,whereas the adsorption tower B′ in the second cleaning step is cleanedwith remaining gas containing a relatively low concentration ofunnecessary gas component. This is because the adsorption tower in thesecond cleaning step has already been cleaned to a higher degree thanthe adsorption tower in the first cleaning step.

[0099] Now, the adsorption tower C′ in the steps S1-S4 is to be noted.Through the four steps, a (second) desorption step, a (first) cleaningstep, a (third) desorption step and a (second) cleaning step areperformed in the adsorption tower C′ in the mentioned order. When thedesorption step of the step S1 is finished, remaining gas containing theunnecessary gas component exists in the adsorption tower C′. In the stepS2, the remaining gas is discharged out of the adsorption tower C′ byremaining gas introduced from the adsorption tower D′. As describedabove, the remaining gas introduced in the step S2 contains a higherconcentration of unnecessary gas component than that introduced into theadsorption tower B′ in the step S1. However, the concentration of theunnecessary gas component in the remaining gas introduced in the step S2is still lower than that of the remaining gas existing in the adsorptiontower C′ when the step S1 is finished. Therefore, the cleaning of theadsorption tower C′ in the step S2 is effective, so that theconcentration of the unnecessary gas component in the adsorption towerC′ after finishing the step S2 is lower than that before the cleaning.As a result, the partial pressure of the unnecessary gas component inthe adsorption tower C′ decreases, which promotes the desorption of theunnecessary gas component from the adsorbent. This is advantageous forreliably regenerating the adsorbent in the third desorption step of S3.In the step S4, remaining gas in the adsorption tower C′ is dischargedby introducing remaining gas from the adsorption tower A′ which isundergoing the first decompression step. Since the concentration of theunnecessary gas component in this remaining gas is relatively low, it ispossible to further reduce the concentration of the unnecessary gascomponent in the adsorption tower C′. As a result, a larger amount ofunnecessary gas component is desorbed from the adsorbent in theadsorption tower C′. In this way, the regeneration efficiency of theadsorbent is greatly enhanced.

[0100] According to the third embodiment, each of the adsorption towersis in the idle state after a second cleaning step. Specifically, theadsorption tower A′ in the step S11, the adsorption tower B′ in the stepS2, the adsorption tower C′ in the step S5 and the adsorption tower D′in the step S8 are in the idle state. According to the presentinvention, an additional cleaning step may be provided for eachadsorption tower in the idle state (See broken lines in FIGS. 8B, 8E, 8Hand 8K). In the case where such an additional cleaning step isperformed, the open/close state of the automatic valves is partiallychanged as indicated in parentheses in FIG. 7. For example, when theadditional cleaning is performed for the adsorption tower B′ in the stepS2, the automatic valves d′, 1′ and u′ are kept open.

[0101] As shown in FIGS. 8B, 8E, 8H and 8K, the additional cleaning isperformed using product gas as cleaning gas. The concentration of theunnecessary gas component in the product gas is lower than that in theremaining gas introduced in the second cleaning step. Therefore, theadditional cleaning can further clean the interior of the adsorptiontower. Further, since the additional cleaning step is performed afterthe first and the second cleaning step, it requires only a relativelysmall amount of cleaning gas (product gas).

[0102] Next, the effectiveness of the present invention will bedescribed using the following Examples 1-6.

[0103] In Examples 1-5, the separation method (the present invention)shown in FIG. 2 was utilized for separating hydrogen gas from materialgas. In Example 6, the separation method (prior art) shown in FIG. 9 wasutilized for separating hydrogen gas from material gas. In all of theExamples 1-6, the separation apparatus X as shown in FIG. 1 was used forperforming the separation.

[0104] As described above, the separation apparatus X includes threeadsorption towers. Each of the adsorption towers has a cylindricalconfiguration having a diameter of 50 mm. The adsorbent used containedzeolite molecular sieve (Ca5A type) and carbon molecular sieve in theratio of 1:1.3 by volume. 2.935 liters of the adsorbent was loaded ineach of the adsorption towers. The material gas used contained 77.77%hydrogen gas, 19.62% carbon dioxide gas, 1% carbon monoxide gas, 0.0008%nitrogen gas and 1.61% methane gas by volume. The material gas wasintroduced at 851 Nliters/hr.

EXAMPLE 1

[0105] In Example 1, the maximum pressure during the adsorption step wasset to 0.95 MPa, whereas the minimum pressure during the desorption stepwas set to be approximately equal to atmospheric pressure (0.106 MPa).The final pressure during the decompression step was set to 0.45 MPa. Asdescribed above, in the separation method of FIG. 2, remaining gas in anadsorption tower in which adsorption has been finished is introduced, ascleaning gas, into another adsorption tower to be cleaned. In Example 1,the introduction amount of the cleaning gas was set to be about 5 timesthe volume of the adsorbent (2.935 liters).

[0106] As a result of the experiment of Example 1, 503 Nliters/hr ofhydrogen gas recovered, and the purity of the hydrogen gas was 99.999vol %. The yield of hydrogen gas was 76.0%.

EXAMPLE 2

[0107] In Example 2, the final pressure during the decompression stepwas set to 0.75 MPa. In the cleaning step, cleaning gas (remaining gas)was introduced in an amount approximately twice the amount of theadsorbent loaded. Other conditions were the same as those of Example 1.

[0108] As a result of the experiment of Example 2, 468 Nliters/hr ofhydrogen gas was recovered, and the purity of the hydrogen gas was99.999 vol %. The yield of hydrogen gas was 70.7%.

EXAMPLE 3

[0109] In Example 3, the final pressure during the decompression stepwas set to 0.55 MPa. In the cleaning step, cleaning gas (remaining gas)was introduced in an amount approximately 4 times the amount of theadsorbent loaded. Other conditions were the same as those of Example 1.

[0110] As a result of the experiment of Example 3, 496 Nliters/hr ofhydrogen gas recovered, and the purity of the hydrogen gas was 99.999vol %. The yield of hydrogen gas was 75.0%.

EXAMPLE 4

[0111] In Example 4, the final pressure during the decompression stepwas set to 0.35 MPa. In the cleaning step, cleaning gas (remaining gas)was introduced in an amount approximately 6 times the amount of theadsorbent loaded. Other conditions were the same as those of Example 1.

[0112] As a result of the experiment of Example 4, 496 Nliters/hr ofhydrogen gas was recovered and the purity of the hydrogen gas was 99.999vol %. The yield of hydrogen gas was 75.0%.

Example 5

[0113] In Example 5, the final pressure during the decompression stepwas set to 0.25 MPa. In the cleaning step, cleaning gas (remaining gas)was introduced in an amount approximately 7 times the amount of theadsorbent loaded. Other conditions were the same as those of Example 1.

[0114] As a result of the experiment of Example 5, 492 Nliters/hr ofhydrogen gas was recovered, and the purity of the hydrogen gas was99.999 vol %. The yield of hydrogen gas was 74.4%.

EXAMPLE 6

[0115] In Example 6, hydrogen gas was separated from the material gasutilizing the method (prior art method including a pressure equalizationstep) shown in FIG. 9, as described above. Therefore, product gasobtained from an adsorption tower during an adsorption step was utilizedas cleaning gas. Cleaning gas (product gas) was introduced in an amountapproximately 0.7 time the amount of the adsorbent loaded. Otherconditions were the same as those of Example 1.

[0116] As a result of the experiment of Example 6, 463 Nliters/hr ofhydrogen gas was recovered, and the purity of the hydrogen gas was99.999 vol %. The yield of hydrogen gas was 70.0%.

[0117] As is clear from the above, as compared with the prior art method(Example 6), the recovered amount and yield of hydrogen gas (productgas) is enhanced in the cases (Examples 1-5) where remaining gas is usedas cleaning gas and the amount of cleaning gas introduced is 2 to 7times the amount of loaded adsorbent.

[0118] In this way, the present invention can enhance the yield oftarget gas while using an adsorbent similar to those used in the priorart method. However, for removing both carbon dioxide and carbonmonoxide at the same time, two kinds of adsorbents (activatedcarbon-based adsorbent and zeolite) need be used. As pointed out before,this causes an increase in the amount of adsorbents to be loaded in eachadsorption tower and hence increases the size of the separationapparatus.

[0119] As a result of the inventors' intensive study for solving theabove-described problem, it is found that the use of zeolite having aparticular structure described below makes it possible to adsorb bothcarbon dioxide and carbon monoxide without using activated carbon-basedadsorbent.

[0120] Specifically, zeolite according to the present invention has afaujasite structure and has a Si/Al ratio of 1-1.5 and alithium-exchange ratio of no less than 95%. The object to be exchangedwith lithium may be Na ion which is a component of zeolite.

[0121] The zeolite is obtained in the following manner.

[0122] First, aluminate solution and silicate solution are mixedhomogeneously. After maturing at 40-60° C. for 20-50 hours, the mixedsolution is crystallized at 90-100° C. for 2-5 hours. Subsequently, thecrystal thus formed is separated from the solution by filtration andthen washed with distilled water. The washed crystal is dried at 70-100°C. and then baked at 500-600° C. for several hours. As a result, zeolitehaving a faujasite structure and has a Si/Al ratio of 1-1.5 is obtained.Specifically, as aluminate, use may be made of sodium aluminate orpotassium aluminate, for example. As silicate, use may be made of sodiumsilicate, for example.

[0123] Subsequently, the zeolite thus prepared is immersed in 0.5-5M oflithium chloride solution held at 70-100° C. for ion exchange. Thezeolite is then washed with dilute solution of lithium hydroxide. Byrepeating such process steps a plurality of times, zeolite with alithium-exchange ratio of no less than 95% is obtained.

[0124] Description will be given below as to the advantages of the useof the zeolite according to the present invention in the PSA process.

[0125] First, reference is made to FIG. 11. This figure schematicallyillustrates the structure of a separation apparatus X″ used forseparating target gas from material gas by the PSA process.Specifically, the separation apparatus X″ includes three adsorptiontowers A-C, a product gas collector 1, a material gas supply 2, and adesorbed gas collector 3. The signs “Gmat”, “Gpro” and “Gdes” in thefigure indicate material gas, product gas and desorbed gas,respectively.

[0126] The adsorption towers A, B and C include product gas outlets Aa,Ba and Ca, respectively, and further include material gas inlets Ab, Bband Cb, respectively. Each of the adsorption towers A-C is loaded withan adsorbent.

[0127] The product gas outlets Aa, Ba and Ca of the adsorption towers A,B and C are connected to the product gas collector 1 through automaticvalves 5A-5C and a pipe 4 a. The pipe 4 a, which is for product gascollection, is provided with a product gas flowmeter 1 a.

[0128] The product gas outlets Aa, Ba and Ca are connected to theproduct gas collection pipe 4 a via pipes 4 b and 4 c. The pipe 4 b,which is for pressure equalization gas and pressurization gas, isprovided with automatic valves 6A-6C. The pipe 4 c, which is forpressurization gas, is provided with an automatic valve 6 a. The productgas outlets Aa, Ba and Ca are connected to the product gas collectionpipe 4 a also via pipes 4 d and 4 e. The pipe 4 d, which is for pressureequalization gas/cleaning gas, is provided with automatic valves 7A-7C.The pipe 4 e, which is for cleaning gas, is provided with an automaticvalve 6 b. The product gas outlets Aa, Ba and Ca are connected to eachother via the pipes 4 b, 4 d and a pipe 4 f for pressure equalization.The pipe 4 f is provided with an automatic valve 6 c.

[0129] The material gas inlets Ab, Bb and Cb of the adsorption towers A,B and C are connected to the material gas supply 2 via a material gassupply pipe 4 g. The pipe 4 g is provided with automatic valves 8A-8Cand a material gas flowmeter 2 a. The material gas inlets Ab, Bb and Cbare connected also to the desorbed gas collector 3 through a desorbedgas collection pipe 4 h. The pipe 4 h is provided with automatic valves9A-9C.

[0130] The gas flow in each of the pipes 4 a-4 h is controlled byappropriately opening or closing each of the automatic valves 5A-5C,6A-6C, 6 a-6 c, 7A-7C, 8A-8C and 9A-9C. As a result, an adsorption step,a first pressure equalization step (decompression step), a desorptionstep, a cleaning step, a second pressure equalization step(pressurization step) and pressurization step are repetitively performedin each of the adsorption towers A, B and C. In the adsorption step,adsorption of unnecessary gas components to the adsorbent is performedunder high pressure. In the first pressure equalization step(decompression step) and the second pressure equalization step(pressurization step), introduction or discharge of gas is performedbetween adsorption towers. In the desorption step, unnecessary gascomponents are desorbed from the adsorbent. In the cleaning step,desorbed gas remaining in an adsorption tower is discharged. In thepressurization step, pressure in an adsorption tower is raised aspreparation for an adsorption step.

[0131] The above-described five steps (an adsorption step, a pressureequalization step, a desorption step, a cleaning step and apressurization step) are performed in each of the adsorption towers attiming shown in the table of FIG. 13. The signs “Ad.”, “PE”, “Des.”,“Cl.” and “Pres.” in the figure indicate an adsorption step, a pressureequalization step, a desorption step, a cleaning step and apressurization step, respectively.

[0132] In the section marked with “V.O.”, reference signs of automaticvalves which are kept open are described. For example, in the first step(S1), an adsorption step (Ad.) is performed in the adsorption tower A,whereas a pressure equalization step (PE) is performed in the adsorptiontowers B and C. At that time, the valves 5A, 8A, 6B, 7C and 6C are openwhereas other valves are closed. The second step (S2) through the ninthstep (S9) are likewise represented in the table.

[0133] FIGS. 12A-12I illustrate gas flow in the first step (S1) throughthe twelfth step (S12).

[0134] As described above, in the first step (S1), an adsorption step isperformed in the adsorption tower A, whereas a pressure equalizationstep is performed in the adsorption towers B and C. The gas flow in thefirst step is illustrated in FIG. 12A. In the adsorption tower A, theproduct gas outlet Aa is held in communication with the product gascollector 1, whereas the material gas inlet Ab is held in communicationwith the material gas supply 2. Material gas is supplied from thematerial gas supply 2 to the material gas inlet Ab through the pipe 4 g.Unnecessary gas components including carbon dioxide are removed in theadsorption tower A and product gas is outputted through the product gasoutlet Aa. The product gas is collected in the product gas collector 1through the pipe 4 a. The amount of material gas supplied to theadsorption tower A is monitored by the material gas flowmeter 2 a foradjusting the flow rate. The amount of product gas collected in theproduct gas collector 1 is monitored by the product gas flowmeter 1 afor adjusting the flow rate. The supply pressure of material gas may be0.5-4 MPa, whereas the recovery pressure of product gas may be 0.4-3.9MPa, for example.

[0135] The adsorption tower B is held in communication with theadsorption tower C via the product gas outlets Ba and Ca. The pressurein the adsorption tower C is relatively high due to the adsorption steppreviously performed therein, whereas the pressure in the adsorptiontower B is relatively low due to the cleaning step previously performedtherein (See the ninth step). In the first step, therefore, remaininggas in the adsorption tower C is discharged through the product gasoutlet Ca and introduced into the adsorption tower B through the pipes 4d, 4 f, 4 b and the product gas outlet Ba. As a result, the pressure inthe adsorption tower C drops, whereas the pressure in the adsorptiontower B increases. (That is, pressure equalization is provided betweenthe adsorption towers B and C.)

[0136] In the second step (S2), the automatic valves 5A, 8A, 6B, 9C and6 a are kept open to realize the gas flow shown in FIG. 12B. Anadsorption step, a pressurization step and a desorption step areperformed in the adsorption tower A, B and C, respectively. In thesecond step, the product gas outlet Aa of the adsorption tower A is heldin communication with the product gas collector 1 and the adsorptiontower B, whereas the product gas inlet Ab is held in communication withthe material gas supply 2. In the second step, product gas is obtainedsimilarly to the first step. However, part of the product gas isintroduced into the adsorption tower B through the pipes 4 c and 4 b aspurge gas (pressurization gas). At that time, the material gas inlet Bbof the adsorption tower B is closed, so that the pressure in theadsorption tower B is increased by the purge gas introduced from theadsorption tower A.

[0137] The product gas outlet Ca of the adsorption tower C is keptclosed. The material gas inlet Cb of the adsorption tower C is held incommunication with the desorbed gas collector 3. The pressure in theadsorption tower C has dropped to some degree by the pressureequalization step performed in the first step. Therefore, unnecessarygas components are desorbed from the adsorbent in the adsorption towerC. At the same time, remaining gas is discharged through the materialgas inlet Cb and collected in the desorbed gas collector 3 through thepipe 4 h. The discharge of remaining gas causes the internal pressure ofthe adsorption tower C to further drop, which promotes desorption ofunnecessary gas components from the adsorbent.

[0138] In the third step (S3), the automatic valves 5A, 8A, 6B, 7C, 9C,6 a and 6 b are kept open to realize the gas flow shown in FIG. 12C. Anadsorption step, a pressurization step and a cleaning step are performedin the adsorption towers A, B and C, respectively.

[0139] In the third step, the product gas outlet Aa of the adsorptiontower A is held in communication with the product gas collector 1, theadsorption tower B and the adsorption tower C, whereas the material gasinlet Ab is held in communication with the material gas supply 2.Therefore, in the third step, product gas is obtained similarly to thefirst step, but part of the product gas is supplied to the adsorptiontowers B and C. In the adsorption tower B, a pressurization step isperformed similarly to the second step. The product gas outlet Ca of theadsorption tower C is held in communication with the product gas outletAa of the adsorption tower A, whereas the material gas inlet Cb is heldin communication with the desorbed gas collector 3. Therefore, theproduct gas is introduced to the product gas outlet Ca of the adsorptiontower C through the pipes 4 e and 4 d as cleaning gas. As a result,remaining gas in the adsorption tower C is collected in the desorbed gascollector 3 through the pipe 4 h.

[0140] In the fourth step (S4), the automatic valves 7A, 5B, 8B, 6C and6 c are kept open to realize the gas flow shown in FIG. 12D. A pressureequalization step, an adsorption step and a pressure equalization stepare performed in the adsorption towers A, B and C, respectively. Thepressure equalization step in the adsorption tower A is performedsimilarly to the pressure equalization (decompression) step performed inthe adsorption tower C in the first step. The adsorption step in theadsorption tower B is performed similarly to the adsorption stepperformed in the adsorption tower A in the first step. The pressureequalization step in the adsorption tower C is performed similarly tothe pressure equalization (pressurization) step performed in theadsorption tower B in the first step.

[0141] In the fifth step (S5), the automatic valves 9A, 5B, 8B, 6C and 6a are kept open to realize the gas flow shown in FIG. 12E. A desorptionstep, an adsorption step and a pressurization step are performed in theadsorption towers A, B and C, respectively. The desorption step in theadsorption tower A is performed similarly to the desorption stepperformed in the adsorption tower C in the second step. The adsorptionstep in the adsorption tower B is performed similarly to the adsorptionstep performed in the adsorption tower A in the second step. Thepressurization step in the adsorption tower C is performed similarly tothe pressurization step performed in the adsorption tower B in thesecond step.

[0142] In the sixth step (S6), the automatic valves 7A, 9A, 5B, 8B, 6C,6 a and 6 b are kept open to realize the gas flow shown in FIG. 12F. Acleaning step, an adsorption step and a pressurization step areperformed in the adsorption towers A, B and C, respectively. Thecleaning step in the adsorption tower A is performed similarly to thecleaning step performed in the adsorption tower C in the third step. Theadsorption step performed in the adsorption tower B is performedsimilarly to the adsorption step performed in the adsorption tower A inthe third step. The pressurization step performed in the adsorptiontower C is performed similarly to the pressurization step performed inthe adsorption tower B in the third step.

[0143] In the seventh step (S7), the automatic valves 6A, 7B, 5C, 8C and6 c are kept open to realize the gas flow shown in FIG. 12G. A pressureequalization step is performed in the adsorption towers A and B, whereasan adsorption step is performed in the adsorption towers C. The pressureequalization step in the adsorption tower A is performed similarly tothe pressure equalization (pressurization) step performed in theadsorption tower B in the first step. The pressure equalization step inthe adsorption tower B is performed similarly to the pressureequalization (decompression) step performed in the adsorption tower C inthe first step. The adsorption step in the adsorption tower C isperformed similarly to the adsorption step performed in the adsorptiontower A in the first step.

[0144] In the eighth step (S8), the automatic valves 6A, 9B, 5C, 8C and6 a are kept open to realize the gas flow shown in FIG. 12H. Apressurization step, a desorption step and an adsorption step areperformed in the adsorption towers A, B and C, respectively. Thepressurization step in the adsorption tower A is performed similarly tothe pressurization step performed in the adsorption tower B in thesecond step. The desorption step in the adsorption tower B is performedsimilarly to the desorption step performed in the adsorption tower C inthe second step. The adsorption step in the adsorption tower C isperformed similarly to the adsorption step performed in the adsorptiontower A in the second step.

[0145] In the ninth step (S9), the automatic valves 6A, 7B, 9B, 5C, 8C,6 a and 6 b are kept open to realize the gas flow shown in FIG. 12I. Apressurization step, a cleaning step and an adsorption step areperformed in the adsorption towers A, B and C, respectively. Thepressurization step in the adsorption tower A is performed similarly tothe pressurization step performed in the adsorption tower B in the thirdstep. The cleaning step in the adsorption tower B is performed similarlyto the cleaning step performed in the adsorption tower C in the thirdstep. The adsorption step in the adsorption tower C is performedsimilarly to the adsorption step performed in the adsorption tower A inthe third step.

[0146] By repetitively performing the above-described first (S1) throughthe ninth (S9) steps, product gas from which unnecessary gas componentshave been removed is provided.

[0147] In the desorption steps and the cleaning steps described above,desorbed gas released from the adsorbent and remaining as in theadsorption towers are collected into the desorbed as collector 3.However, in the case where the desorbed gas and the remaining gas areless toxic, it is possible to release these gases into the atmosphere.

[0148] According to the experiments by the inventors of the presentinvention, the adsorption isotherm (25° C.) of zeolite (withlithium-exchange ratio of no less than 95%) according to the presentinvention becomes as shown in the graph of FIG. 14. This graph isobtained by adding the adsorption isotherm of the zeolite according tothe present invention to the graph of FIG. 17 referred to before. As isclear from the graph of FIG. 14, in the case of the zeolite with 95%lithium-exchange ratio, the adsorption amount of carbon dioxide at theequilibrium adsorption pressure of 180 Torr is 52 ml/g while theadsorption amount of carbon dioxide at the equilibrium adsorptionpressure of 1440 Torr is 116 ml/g. This indicates that 64(=116-52) ml/gof carbon dioxide gas can be removed by varying the equilibriumadsorption pressure in the range of 180 to 1440 Torr. This value ishigher than those of the amount of carbon dioxide (37 ml/g and 48 ml/g)removed by the prior art adsorbent described with reference to FIG. 17.

[0149] Similarly to conventional zeolite-based adsorbents, the zeolitewith 95% lithium-exchange ratio according to the present invention iseffective for removing carbon monoxide gas or nitrogen gas. The graph ofFIG. 15 shows the adsorption isotherm (25° C.) of three kinds ofzeolite-based adsorbents for carbon monoxide gas. As is clear from thegraph, when the zeolite (with 95% lithium-exchange ratio) according tothe present invention is used, carbon monoxide gas can be effectivelyremoved by varying the partial pressure of monoxide in the range of0.001 MPa (7.5 Torr) to 0.008 MPa (60 Torr). FIG. 16 is a graph showingthe adsorption isotherm (25° C.) of the three kinds of zeolite-basedadsorbents for methane gas.

[0150] As described above, both of carbon dioxide and carbon monoxidecan be removed in the PSA process by the use of 95% lithium-exchangeratio zeolite having a faujasite structure. Since only a single kind ofadsorbent is used, the size of the entire apparatus is prevented fromunduly increasing. According to the present invention, thelithium-exchange ratio is not limited to 95%, but may be any value inthe range of 95 to 100%.

[0151] The zeolite with 85% lithium-exchange ratio and that with 95%lithium-exchange ratio which have adsorption characteristic as shown inFIG. 14 can be obtained in the following manner.

[0152] For preparing a lithium-exchange zeolite, zeolite having afaujasite structure with a Si/Al ratio of 1 is first prepared.Specifically, solution of sodium/potassium aluminate and solution ofsodium silicate are mixed homogeneously. After maturing at 50° C. for 30hours, the mixed solution is crystallized at 95° C. for three hours.Subsequently, after undergoing filtration, the crystal is washed withdistilled water, dried at 80° C. and baked at 550° C. for two hours. Asa result, zeolite having a faujasite structure and a Si/Al ratio of 1 isobtained.

[0153] As the solution of sodium/potassium aluminate, use is made of oneprepared by dissolving 15.6 g of gibbsite-type alumina trihydrate in asolution containing 100 ml of water, 33.68 g of sodium hydroxide pelletsand 17.92 g of potassium hydroxide pellets at 100-115° C. followed bycooling the solution to 20° C. and making up for water lost byevaporation through the dissolution. As the sodium silicate solution,use is made of one prepared by dissolving 47.05 g of sodium silicate(SiO₂/Na₂O=25.5:7.75) in 100 ml of water.

[0154] After the zeolite thus obtained is immersed in lithium chloridesolution for ion-exchange, the ion-exchanged material is washed withdilute solution of lithium hydroxide. To obtain zeolite with 85%lithium-exchange ratio, the combined operation of ion exchange andwashing is repeated twice using lithium chloride solution having aconcentration of 3M. To obtain zeolite with 95% lithium-exchange ratio,the combined operation of ion exchange and washing is repeated threetimes using lithium chloride solution having a concentration of 4M.

[0155] For lithium-exchange zeolite obtained in this way, thelithium-exchange ratio is calculated by 100xLi₂O/(Li₂O+Na₂O). That is,the ion-exchange ratio is represented by the ratio of the number ofmetal ions actually exchanged with Li ions to the number of metal ionscapable of being replaced with Li ions.

[0156] A comparison is given below between the case where a 95%lithium-exchange zeolite is solely used as the adsorbent and the casewhere a Ca A-type zeolite and a carbon-based adsorbent are used asadsorbents.

EXAMPLE A

[0157] In Example A, the cycle consisting of the steps shown in thetable of FIG. 13 was repeated by the separation apparatus X″ as shown inFIG. 11 under the conditions described below.

[0158] One liter of adsorbent was loaded in each of the adsorptiontowers A, B and C. As the adsorbent, use was made of zeolite with 95%lithium-exchange ratio (having a faujasite structure with a Si/Al ratioof 1).

[0159] Material gas having a composition (by volume) consisting of 75%hydrogen gas, 24% carbon dioxide gas and 1% carbon monoxide gas wassupplied to an adsorption tower during adsorption at 0.5 Nm³/hr. Thematerial gas maybe obtained by steam-reforming a hydrocarbon-basedcompound. Alternatively, the material gas may be obtained bysteam-reforming a hydrocarbon-based compound followed by conversion ofcarbon monoxide contained in the reformed gas (thereby reducing thecontent of carbon monoxide)

[0160] In each of the adsorption towers A, B and C, the final pressureduring the adsorption step and that during the desorption step were setto 0.8 MPa and atmospheric pressure, respectively. Each of the stepsshown in the table of FIG. 13 was performed for 300 seconds.

[0161] As a result, 0.28 Nm³/hr of hydrogen gas was obtained as productgas. The recovery rate of hydrogen gas was 75%.

EXAMPLE B

[0162] In Example B, the cycle consisting of the steps shown in thetable of FIG. 13 was repeated by the separation apparatus X″ under theconditions described below.

[0163] A Ca-exchange A-type zeolite (Tradename:Zeolum, available fromTOSOH CORPORATION) for carbon monoxide removal and a carbon-basedzeolite (Tradename:CMS, available from Carbo Tech Aktivekohlen GmbH) forcarbon dioxide removal are loaded in each of the adsorption towers A-Cat a ratio of 50:50 by volume to be one liter in total. The material gaswas supplied at 0.28 Nm³/hr.

[0164] As a result, 0.14 Nm³/hr of hydrogen gas was obtained as productgas. The recovery rate of hydrogen gas was 67%.

[0165] As will be understood from the above, when the 95%lithium-exchange zeolite is solely used (Example A), the recovery ofhydrogen gas is higher than in the case where two kinds of adsorbentsare used (Example B) for removing carbon monoxide and carbon dioxide.Further, the amount of supplied material gas in the method of Example Ais more than that in the method of Example B. Therefore, the method ofExample A can remove carbon dioxide more effectively than the method ofExample B (prior art), and can be suitably utilized not only forremoving carbon monoxide but for removing carbon dioxide. Therefore, theamount of adsorbent to be used can be reduced.

[0166] The present invention being thus described, it is apparent thatthe same may be varied in many ways. Such variations should not beregarded as a departure from the spirit and scope of the presentinvention, and all such modifications as would be obvious to thoseskilled in the art are intended to be included within the scope of thefollowing claims.

1. A method for separating hydrogen gas from material gas usingplurality of adsorption towers each of which is loaded with an adsorbentand is provided with a product gas outlet, the method comprising: anadsorption step for adsorbing an unnecessary gas component contained inthe material gas by the adsorbent for outputting hydrogen-rich productgas through the product gas outlet; a decompression step for reducingpressure in an adsorption tower; a desorption step for desorbing theunnecessary gas component from the adsorbent; a cleaning step forcleaning the adsorption tower by introducing cleaning gas into theadsorption tower; and a pressurizing step for raising pressure in theadsorption tower; one cycle comprising the adsorption step, thedecompression step, the desorption step, the cleaning step and thepressurization step being repeated; wherein the decompression stepincludes introducing gas remaining in the adsorption tower into aselected adsorption tower as cleaning gas, the remaining gas beingintroduced in an amount 2 to 7 times a volume of the adsorbent loaded inthe selected adsorption tower as converted into volume at commontemperature and under atmospheric pressure.
 2. The method according toclaim 1, wherein the cleaning step includes an additional cleaning stepperformed by introducing product gas obtained from an adsorption towerundergoing the adsorption step as cleaning gas.
 3. The method accordingto claim 1, wherein the cleaning step in said one cycle includes a firstcleaning step and a second cleaning step performed after the firstcleaning step, the decompression step in said one cycle including afirst decompression step and a second decompression step performed afterthe first decompression step, the first and the second decompressionsteps being performed by discharging remaining gas through the productgas outlet.
 4. The method according to claim 3, wherein the firstcleaning step in a first adsorption tower is performed by introducingtherein remaining gas discharged from a second adsorption tower duringthe second decompression step through the product gas outlet of thefirst adsorption tower as cleaning gas, the second cleaning step in thefirst adsorption tower being performed by introducing therein remaininggas discharged from a third adsorption tower during the firstdecompression step through the product gas outlet of the firstadsorption tower as cleaning gas.
 5. The method according to claim 4,wherein the cleaning step includes a third cleaning step performed byintroducing product gas obtained from an adsorption tower undergoing theadsorption step as cleaning gas.
 6. The method according to claim 5,wherein, in said one cycle, the decompression step, the desorption step,the first cleaning step, the desorption step, the second cleaning stepand the third cleaning step are performed in the mentioned order in eachof the adsorption towers.
 7. The method according to claim 1, whereinmaximum pressure in the adsorption step lies in a range of 0.2 to 3.6MPa (absolute pressure), whereas minimum pressure in the desorption steplies in a range of atmospheric pressure to 0.15 MPa (absolute pressure).8. The method according to claim 1, wherein the material gas containscarbon dioxide gas as the unnecessary gas component.
 9. A method forremoving at least carbon dioxide gas from material gas to obtain targetgas, the method comprising: an adsorption step for introducing thematerial gas into an adsorption tower loaded with an adsorbent forremoving an unnecessary gas component including carbon dioxide by theadsorbent; and a desorption step for reducing pressure in the adsorptiontower to separate the unnecessary gas component from the adsorbent; onecycle including the adsorption step and the desorption step; whereinminimum pressure in the desorption step is set to be approximately equalto atmospheric pressure; the adsorbent comprising zeolite having afaujasite structure with a Si/Al ratio lying in a range of 1 to 1.5 andlithium-exchange ratio of no less than 95%.
 10. The method according toclaim 9, wherein maximums pressure in the adsorption step lies in arange of 0.5 to 4 MPa (absolute pressure).
 11. The method according toclaim 9, wherein the material gas is a gas obtained by steam-reforming ahydrocarbon-based compound and contains carbon dioxide gas and hydrogengas.
 12. The method according to claim 11, wherein the gas obtained bysteam-reforming contains carbon monoxide, the material gas beingobtained after a content of the carbon monoxide is reduced byconversion.
 13. A gas separation apparatus comprising: an adsorptiontower for introducing material gas; and an adsorbent loaded in theadsorption tower to remove unnecessary gas including carbon dioxide gasfrom the material gas; wherein the adsorbent comprising zeolite having afaujasite structure with a Si/Al ratio lying in a range of 1 to 1.5 anda lithium-exchange ratio of no less than 95%.